1. Technical Field
The present invention relates to an actuator, a method for manufacturing the actuator, and an optical scanner.
2. Related Art
There is a known actuator using a structure including a torsional oscillator formed by processing a silicon substrate with the aid of MEMS (micro-electro-mechanical systems) technology (see JP-A-7-175005, for example). The actuator is used as an optical scanner that deflects light in a printer, a display, or other apparatus.
For example, the actuator described in JP-A-7-175005 includes a flat movable plate and a pair of torsional bars that pivotally support the movable plate. The actuator further includes a flat coil provided over the movable plate and fixed permanent magnets, and the movable plate oscillates based on magnetic interaction between the flat coil and the permanent magnets.
In the actuator, the flat coil is disposed over the movable plate with an insulating layer therebetween so that the flat coil and the movable plate are electrically insulated from each other.
The structure including the movable plate and the pair of torsional bars is formed by etching a silicon substrate.
When the structure is formed in a dry etching process, irregularities called scallops are disadvantageously formed on the side surfaces of the movable plate and the torsional bars. On the other hand, when the structure is formed in a wet etching process, corners resulting from the silicon crystal plane are disadvantageously formed along the interface between the movable plate and each of the torsional bars.
When such irregularities and corners are formed on the torsional bars, stress concentration tends to occur when the movable plate oscillates, resulting in decrease in lifetime of the actuator. It is therefore necessary to planarize the irregular side surfaces and round the corners. To this end, it is effective to perform a thermal treatment using silicon surface diffusion motion. In the actuator described in JP-A-7-175005, however, an insulating layer formed of a silicon oxide film is also formed in the vicinity of the side surfaces of the movable plate and the torsional bars. The insulating layer disadvantageously prevents the irregular side surfaces from being fully planarized and the corners from being fully rounded.